[50mhz] S-Meter readings, was Re: [VHF] Preamp and receiver question

[kd7ts]

I did quite a bit of searching and reading due to a recent interest in  
dbm, logarithms and micro volts.

All S-Meter systems seem to derive from work done in the 1940s by an  
unnamed receiver manufacturer. I seem to recall reading it was National,  
but my memory does not serve well lately, and is not likely to improve.

The system was originally based on S-9 being equvalent to 50 microvolts at  
the antenna terminals and each halving of voltage is the next lower unit.  
So S-8 is 25 micro volts, S-7 is 12.5 micro volts etc..

Signal levels above S-9 are labeled in db above S-9. Clever how db and  
microvolts are incorporated on the same meter face.

There appear to be at least 3 methods of this system. All are based on the  
signal in microvolts for S-9.

The original as mentioned above. Another that uses S-9 equals 70  
microvolts and a third that says it is for frequencies above 30 megahertz  
and has S-9 equal to 5 micro volts. There may be others, but these were  
the ones I found. I did find a reference to the Yaesu 817 by KA7OEI which  
mentions dbu. I haven't pursued that subject, but it is at  
http://www.ka7oei.com/ft817_oops.html for anyone that cares to review the  
Yaesu method.

There is a nice chart and discussion of the original system, and the one  
mentioned for frequencies above 30 megahertz at:

http://www.giangrandi.ch/electronics/radio/smeter/smeter.html

I lost my reference to the 70 micro volt scale, but it is interesting that  
all this stuff is in even dbm. 70 uV is 3db greater than 50 uV and 50 uV  
is 20 db greater than 5 uV. Interesting ! Maybe this has something to do  
with the difference of the ICOM and Kenwood ? Just guessing here.

Both may be correct in their own way. Signal reports should include the  
manufacturer and model of the radio used to receive, possibly the age as  
well.

A great deal of obvious information was omitted to allow comments by those  
so inclined.

73 Mike KD7TS